I am looking for some more info on parti-gyle brewing. I get the process and I have seen some great charts/calculators for 1/3 & 2/3 batches, and for 1/2 & 1/2. What I am looking for is how to calculate for 2/3 big beer, 1/3 small beer. I brew 10 gallon all grain normally, so I would do this as 10 gallons of the bigger one and 5 of the smaller one. I have been trying to figure this out - damn logarithmic sugars - and am stuck, so thanks ahead of time.

Not sure how much I can help because I've only done one partigyle and it was just last weekend. I also did two 5 gal batches but one with an OG of 1.082 and the other at 1.048. And I 'cheated'. But here's what I did.

Basically I had plenty of DME on-hand (by design, I needed to use up a whole 3# bag before it got old). When lautering/sparging, each of the two boil kettles were filled to about a gallon LESS than the preboil volume I needed. Then I just measured the gravity and exact volume of each kettle, did a little math, then added the required amounts of DME and water to hit my target pre-boil gravity and volume in each kettle. Then boiled each one normally from there. I went way smoother than I expected and I hit my volumes/gravities.

I also did a decoction mash and ran out of propane mid-boil...made for a 9 hour brewday. But I like brewing so it was fun.

__________________Early brewers were primarily women, mostly because it was deemed a woman's job. Mesopotamian men, of some 3,800 years ago, were obviously complete assclowns and had yet to realize the pleasure of brewing beer.- Beer Advocate

One thing you could do would be to collect your big beer as a 10 gallon no-sparge batch and your small beer from a batch sparge. You'll need to know your conversion efficiency for this. Once you've calculated the volume V and gravity G you need to collect to hit your desired original gravity for the big beer, you can calculate the necessary mash thickness by first converting G to degrees Plato (call this g), then:

Now you can see that this is maybe not such a great idea: Using this method, assuming 100% conversion and 80% extraction, if we want to collect 12 gallons of 1.092 SG (22 Plato) wort to boil down for a big beer, we will need a mash thickness of 1.36 qts/lb, which translates to 50 lbs. of grain. That's an awful lot, and the calculations for the small beer are even more complex...

Thats the split I usually do. At a mash thickness of ~1.3 qt/lb, I use my 2nd runnings for the small beer (~20-25% efficiency) and 1st & 3rd (~65% eff) for the big beer. You can use kai's batch sparge simulator to test the correct volumes.